1. Field of the Invention
The present invention relates to a multiband planar antenna and electronic equipment equipped with the multiband planar antenna.
2. Background Art
There has previously been known a portable device having a wireless communication function such as a handheld terminal and a Personal Digital Assistant (PDA). The portable device has an antenna for wireless communication.
As the antenna for wireless communication to be mounted on the portable device, there has been known a single band planar antenna (for example, see Japanese Patent Application Laid-Open Publication No. 2004-356823 and Japanese Patent Application Laid-Open Publication No. 2002-55733).
As the single band planar antenna, there has been known a planar antenna in which two flat plates are provided in the same plane so as to face to each other. This planar antenna resonates with one resonance frequency which is determined depending on a length of each of the flat plates.
However, the conventional planar antenna has been a single band planar antenna in which the number of resonance frequency band is one (1). In order to adapt to wireless communication such as mobile communication in which there are a plurality of resonance frequency bands, making the planar antenna become a multiband antenna has been required.
The planar antenna can be the multiband antenna by being provided with an antenna element including a plurality of sides each of which resonates with a different frequency and has a different length from those of other sides. A planar antenna 50 will be explained with reference to FIGS. 18-20.
FIG. 18 shows a schematically configuration of the planar antenna 50 which has two sides having different lengths.
FIG. 19 shows a current distribution of the planar antenna 50.
FIG. 20 shows S parameter with respect to a frequency of the planar antenna 50.
As shown in FIG. 18, the planar antenna 50 including two sides of different lengths will be described. The planar antenna 50 is a flat plate antenna. The planar antenna 50 is equipped with antenna elements 53, 54. The antenna elements 53, 54 are connected to a coaxial cable at a feeding point P. In FIG. 18, details of substrates of the antenna elements 53, 54 and of the feeding point P of the planar antenna 50 are omitted.
The antenna elements 53, 54 are line-symmetric across the feeding point P, each having an L-shape and a planar shape, and face to each other on the same plane. The antenna element 53 has a lower side 531 and an upper side 532. The antenna element 54 has a lower side 541 and an upper side 542. The lengths of the lower sides 531 and 541 are same as each other, each of the length being considered to be length L51. The lengths of the upper sides 532 and 542 are same as each other, each of the length being considered to be length L52. The length L51 and the length L52 have a relation of L51>L52.
Antenna current flowing through the planar antenna 50 when it receives radio wave having a frequency f51 corresponding to the length L51 and a frequency f52 corresponding to the length L52 was simulated. A frequency which corresponds to a wavelength λ51 in case of L51=λ51/4 (λ51: wavelength of radio wave) is considered to be the frequency f51. A frequency which corresponds to a wavelength λ52 in case of L52=λ52/4 (λ52: wavelength of radio wave) is considered to be the frequency f52. Here the values of f51 and f52 are considered such that f51=1.57 [GHz] and f52=1.91 [GHz].
As a result of such simulation, with respect to the frequency f51 and the frequency f52, the same current distribution of the antenna current shown in FIG. 19 was obtained.
In FIG. 19, as the value (Amps/m) of the antenna current changes from low to high, a color changes from black to white. The same can be said for subsequent drawings of current distribution of the antenna current.
In addition, characteristics of S parameter with respect to the frequency of the planar antenna 50 were simulated. The smaller the value of S parameter, the greater the resonance of the antenna. This simulation result shows antenna characteristics in which the number of drops of graph of S parameter was one (1) and its value was 1.57 [GHz], as shown in FIG. 20. Since the number of the drops was one (1), the number of frequency band to resonate was also one (1), and thereby the planar antenna 50 had characteristics of single band antenna.
Thus, it has been impossible to allow the planar antenna to have multiband by merely providing the antenna elements having two sides of different lengths.